The present invention concerns a beam structure for a paper, board or finishing machine. The beam structure of the invention is suited for use in all applications where a device needs to be supported in the cross direction of a paper or board machine so that the said device stays as straight as possible. The beam of the invention is preferably used to support a doctor, or a coating, measuring, or washing device. Especially preferably, the beam structure comprises a beam which is made of a composite material and which is supported at the middle.
In the following, prior art beam solutions are presented using doctor support beams as examples, because they are the most common beam solutions and because their structures have also been applied in many other uses of beams.
Earlier, when paper machines were narrower and when the deflection criteria were less strict, a solid doctor support beam made of steel or other purposeful metal was sufficient. The characteristic feature of these beams is, however, that they cannot usually reach the quality level required by modern and fast paper machines in the control of deflection and potential vibrations.
This is why it was presented in the previous decade that doctor support beams would no longer be made of solid metal beams, but that they could be, for instance, hollow and rigid box-type structures which are potentially made of a composite material. Such doctor support beams are described in publications U.S. Pat. No. 5,356,519; DE-A1-197 13 195; and FI-B105578.
U.S. Pat. No. 5,356,519 describes a doctor support beam which consists of a hollow box-type structure where the cross section of the structure is either an equilateral or inequilateral polygon or oval. According to the publication, the structural material is fiber composite material.
German publication DE-A1-197 13 195 describes a doctor support beam where the load-bearing part consists of a number of tubular elements joined together, with the elements manufactured from a suitable fiber material. All of the said elements can have the same diameter, or they can also have different diameters. The publication also states that in addition to tubes with a round cross section, triangular tubes, for instance, can also be used.
Finnish patent publication FI-B-105578 describes a beam structure where the cross section consists of a curved part and a straight part. The curved part is preferably semicircular, and the straight part encloses it to form a box beam. The doctor blade or similar component is fastened to the point of contact between the curved part and the straight part by means of suitable devices.
The beam structures presented above, however, have not attained much success on the market. The reason for this can be both the complex structure of the beams which makes the beam unreasonably expensive, and their cross-sectional shape which is disadvantageous from the point of view of the manufacture of the beam and also raises the price of the beam. Moreover, a complex structure increases the risk of manufacturing defects considerably and may also lead more easily to breakages resulting from stress during operation.
However, since the beams made of a composite material, presented above, have mostly turned out to be practicable solutions with the exception of issues such as the above-mentioned high manufacturing costs and risks, the present invention has attempted to find a beam structure which could be manufactured inexpensively for instance from composite materials without ignoring the requirements imposed on the beam.
As known, a small deflection (recommended maximum guideline value is half a millimeter irrespective of the length of the beam, in some cases a maximum deflection of up to one millimeter is permitted at the middle of the beam) is one of the most important properties of a beam in all applications of beams. Moreover, especially in the case of doctor support beams, however, the vibration properties of the beam must be taken into account in the design of beams so that when using the conventional subcritical dimensioning, the first natural frequency of the beam would be at least 20 percent above the excitation frequency of the roll. When using supercritical dimensioning, the natural frequency of the beam would therefore be at least that much below the excitation frequency of the roll. Supercritical dimensioning could also be used in conjunction with the present invention because of the damping elements at the ends and because of deflection control so that even smaller beam dimensions could be reached.